The response of the mangrove Avicennia marina to heterogeneous salinity measured using a split-root approach

To investigate the physiological processes underlying freshwater utilisation in halophytes under non-uniform salinity conditions in order to determine whether preferential uptake of freshwater occurs and whether transient freshwater availability improves plant water relations and photosynthesis In a split-root experiment, we grew Avicennia marina (Forssk.) Vierh. at uniform salinity conditions (35:35 ppt) and then lowered (35:5 ppt) or increased (35:65 ppt) salinity to one compartment. Using delta O-18-labelled water we calculated the extent of preferential water uptake of either source. When given a 35:5 treatment, A. marina took-up three times more water from the fresher compartment than predicted by a root-weighted-no-salinity-preference model. No difference between compartments was observed in the 35:65 treatment, suggesting that avoidance of the more saline water did not occur. In the 35:5 treatment, stomatal conductance increased within 1.5 h, but photosynthetic rates were not enhanced over the 48-h period of the experiment and rapidly decreased in the 35:65 treatment. Avicennia marina responds to transient freshwater patches by increasing water uptake from areas of the root zone where the salinity is better for growth. However, photosynthetic rates respond to the highest salinity in the root zone; thus, reductions in salinity in part of the root zone may enhance water storage in stem tissues but positive effects on photosynthesis may require longer periods.